1. Field of the Invention
The present invention relates a black matrix used for a color filter used for a liquid crystal display device or the like and a method for manufacturing the same.
2. Description of the Related Art
A black matrix is a black edge formed on the circumferences of the red, blue and green pixels of a color filter so as to prevent the leaking of light.
In a color filter used for a color liquid crystal display or the like, colored pixel layers (R, G, B) are formed on a transparent substrate, and in order to secure an improvement in display contrast or the like, black matrices are formed in gaps between the colored pixels, R, G, B (red, green, blue). Especially, in an active matrix driving type liquid crystal display device using thin film transistors (TFT), a black matrix having a high light-shielding property (optical density) is required so as to prevent a deterioration in image quality as a result of leakages of current caused by the light of the thin film transistors.
It is possible to use metal to manufacture a black matrix having a high light-shielding property. As technique for manufacturing a black matrix with metal fine particles, a plating technique for forming nickel fine particles in a layer has already been disclosed (see Japanese Patent Application Laid-Open (JP-A) No. 5-303090).
This method, however, has drawbacks, such as complications in precipitating fine particles from a plating solution containing metal ions, complications in the disposal of the waste solution, and many other environmental considerations.
On the other hand, a method of manufacturing a black matrix without using the plating technique is also known, for example, a black matrix formed by using carbon black instead of metal fine particles (see JP-A No. 62-9301).
However, since carbon black has lower optical density per unit of coating amount than that of metal fine particles, if a high light-shielding property and high optical density need to be secured, the film thickness inevitably increases, and once the black matrix has been formed it becomes difficult to form uniform red, blue, and green pixels.
Among additional ways of forming a black matrix by means of a plating technique, a following according to which a metal film such as a chrome film is used as a shading layer and, a thin metal film is then formed by a deposition method or by a sputtering method. A photo resist is then applied on the thin metal film, the photo resist layer is then exposed and developed with the use of a photo mask having patterns for a black matrix, and the thin metal film is exposed and then etched. Finally, the resist layer remaining on the thin metal film is exfoliated, and a black matrix is thereby formed (For example, see “Color TFT liquid crystal display” PP. 218–220, published by Kyoritsu Shuppan Co., Ltd. (Jul. 20, 1996)).
Since a thin metal film is used in this method, a high shading effect can be obtained even when the thickness of the metal film is minimal, however, when a vacuum film deposition process, such as a deposition method or a sputtering method, or an etching process become necessary, problems arises insofar that costs become higher and the burden on the environment can not be lightly dismissed. Moreover, since a metal film invariably has high reflectivity, another problem has been that display contrast is poor in conditions of strong external light. In contrast, another method is to use a low reflection chromium film (a laminated body or the like composed of two layers, a metal chrome layer and an oxidation chrome layer). However, in this case it can not be denied that cost become even higher.
Further, the hue or the reflectivity of a black matrix containing metal fine particles may be changed as a result of exposure to high temperatures during manufacture, and bubbles may be caused. Thus there is a need in the art for forming an improved black matrix for a liquid crystal display.
Furthermore, additional ways of forming a black matrix containing metal sulfide fine particles by means of a plating technique are known (for example, see JP-A No. 7-218715). However, since these methods also use a plating technique, they need environmental considerations about waste solution and complications in processings. Therefore, these methods are not satisfying, too.